1. VISUAL PIGMENTS
9
changes in the retina will be described, together with the tapetum
lucidum. In some species, the tapetum can be occluded by a migration
of black pigment, which is the reason for its mention here. The most
familiar components of light and dark adaptation-changes in concentration of visual pigment and modifications in neuronal interaction-are
described in Chapter 2.
With the possible exception of deep-sea forms, elasmobranchs usually
have a highly mobile pupil. According to Young (1933a), Nicol (1964),
and Kuchnow and Gilbert ( 1967), the pupil closes quite rapidly (2-15
min) after the eye is exposed to light. Dilatation in darkness occurs more
slowly, requiring 30 min or more. In elasmobranchs, Young (1933a)
found that the irideal sphincter contracts in direct response to illumination and is not under nervous control. The dilatator muscle is innervated
by the oculomotor nerve. Anguillu, one of very few teleosts with appreciable mobility of the pupil, has been studied by Seliger ( 1962). Light
has a direct effect on excised pieces of iris, causing the sphincter muscle
to contract. The action spectrum for this effect has a primary maximum
near 500 nm, suggesting that the light may be absorbed by a rhodopsinlike pigment. In darkness the iris relaxes. The iris of Uranoscopus and
Lophius has dual innervation, oculomotor to the dilatator and sympathetic to the sphincter muscles (Young, 1931, 1933b). Some flatfishes have
a mobile pupillary operculum, but not all (Nicol, 1965a).
In the great majority of teleosts, photomechanical (also called “retinomotor”) movements take the place of pupillary control of light intensity
at the retinal level. The main processes were reviewed by Walls (1942),
Brett ( 1957), and Nicol ( 1963) and need only be summarized. The outermost retinal cell layer is called the “pigment epithelium.” Long processes
of the pigment cells extend toward the visual cells and interdigitate with
their outer segments. In the dark-adapted eye, melanin granules within
these pigment cells are drawn back, away from the visual cells (Fig. 3).
Exposure to light is soon followed by migration of pigment granules into
the processes. The visual cells have a contractile “myoid region proximal
to the outer segment. When the rod-cell myoids change length, the outer
segments move counter to the melanin in the pigment epithelium. Under
conditions of dark adaptation, the rod outer segments are pulled proximally, toward the focal plane; and the pigment is retracted. In the lightadapted retina, the expanded pigment surrounds and shields the extended outer segments of the rods. The cone outer segments move also,
in a manner opposite to the rods. They are never shielded by the pigment
epithelium. Perhaps their extension in the dark-adapted retina merely
clears the focal plane for the rod outer segments. In Ameiurus ( = Zctalurus), photomechanical movements follow a die1 cycle (Welsh and
Osborn, 1937).
9
changes in the retina will be described, together with the tapetum
lucidum. In some species, the tapetum can be occluded by a migration
of black pigment, which is the reason for its mention here. The most
familiar components of light and dark adaptation-changes in concentration of visual pigment and modifications in neuronal interaction-are
described in Chapter 2.
With the possible exception of deep-sea forms, elasmobranchs usually
have a highly mobile pupil. According to Young (1933a), Nicol (1964),
and Kuchnow and Gilbert ( 1967), the pupil closes quite rapidly (2-15
min) after the eye is exposed to light. Dilatation in darkness occurs more
slowly, requiring 30 min or more. In elasmobranchs, Young (1933a)
found that the irideal sphincter contracts in direct response to illumination and is not under nervous control. The dilatator muscle is innervated
by the oculomotor nerve. Anguillu, one of very few teleosts with appreciable mobility of the pupil, has been studied by Seliger ( 1962). Light
has a direct effect on excised pieces of iris, causing the sphincter muscle
to contract. The action spectrum for this effect has a primary maximum
near 500 nm, suggesting that the light may be absorbed by a rhodopsinlike pigment. In darkness the iris relaxes. The iris of Uranoscopus and
Lophius has dual innervation, oculomotor to the dilatator and sympathetic to the sphincter muscles (Young, 1931, 1933b). Some flatfishes have
a mobile pupillary operculum, but not all (Nicol, 1965a).
In the great majority of teleosts, photomechanical (also called “retinomotor”) movements take the place of pupillary control of light intensity
at the retinal level. The main processes were reviewed by Walls (1942),
Brett ( 1957), and Nicol ( 1963) and need only be summarized. The outermost retinal cell layer is called the “pigment epithelium.” Long processes
of the pigment cells extend toward the visual cells and interdigitate with
their outer segments. In the dark-adapted eye, melanin granules within
these pigment cells are drawn back, away from the visual cells (Fig. 3).
Exposure to light is soon followed by migration of pigment granules into
the processes. The visual cells have a contractile “myoid region proximal
to the outer segment. When the rod-cell myoids change length, the outer
segments move counter to the melanin in the pigment epithelium. Under
conditions of dark adaptation, the rod outer segments are pulled proximally, toward the focal plane; and the pigment is retracted. In the lightadapted retina, the expanded pigment surrounds and shields the extended outer segments of the rods. The cone outer segments move also,
in a manner opposite to the rods. They are never shielded by the pigment
epithelium. Perhaps their extension in the dark-adapted retina merely
clears the focal plane for the rod outer segments. In Ameiurus ( = Zctalurus), photomechanical movements follow a die1 cycle (Welsh and
Osborn, 1937).